1. Field of the Invention
The invention relates to zinc-containing optical glass materials which have refractive indices nd of from 1.52 to 1.66 and Abbe numbers vd of from 35 to 54.
2. Background Information
During the production of optical glass materials, in addition to an accurate setting of the required optical properties and achieving a high transmission, the production costs and raw materials costs play an important role.
The latter can be achieved in particular by using inexpensive batch raw materials and by glass compositions which require relatively low melting temperatures and therefore entail lower energy costs.
A large proportion of the known glass materials which have the abovementioned optical properties (refractive indices, Abbe numbers) belong to the group of barium flint glasses. To set the desired optical properties and to achieve high transmission, substantially the high-quality and expensive raw materials from the group consisting of the alkaline earth metal oxides (BaO, SrO, CaO) and PbO are used.
Since the glass components BaO and PbO have become the subject of public discussion in terms of the environmental pollution which they cause, the manufacturers of optical equipment need optical glass materials which contain the smallest possible amounts of these components.
Partially or completely substituting one glass component with one or more other glass components does not generally allow the required optical properties to be reproduced. Instead, new developments or far-reaching changes in the glass composition are required.
The patent literature includes a number of documents which already describe glass materials with optical characteristics from the said range and with similar compositions. However, these glass materials exhibit a wide range of drawbacks:
In addition to from 7 to 30% by weight ZnO, the glass materials from Japanese document JP 85-24060 B2, with refractive indices nd of from 1.59 to 1.63 and Abbe numbers vd from 36 to 50, also contain the critical components BaO and PbO, in each case in an amount of from 1 to 21.5% by weight.
The BaO content of from 0.9 to 15.7% by weight which is provided according to SU 1534979 A1 and is absolutely necessary is to be regarded equally critically; these glass materials exhibit a refractive index nd of from 1.577 to 1.634.
Japanese document JP 89-133956 A relates to optical glass materials with refractive indices nd of between 1.51 and 1.59 and Abbe numbers vd of between 39 and 58, which, although only optionally, contain up to 44.06% by weight PbO, up to 25.34% by weight B2O3, up to 19.06% by weight SrO and up to 11.3% by weight CaO. In addition, these glass materials contain relatively large amounts of Al2O3 ranging from 4.45 to 44.10% by weight, which entails an increase in the melting temperature. Also, from 0.08 to 8.0% by weight F is absolutely imperative.
Moreover, the glass materials may contain in total up to 11.1% by weight As2O3 and Sb2O3. When these components are used in such high quantities, increased safety provisions are required during production.
At nd of xe2x89xa61.62, the glass materials from document JP 89-308843 A have relatively high refractive indices, while their Abbe numbers are relatively low at vdxe2x89xa637. In addition to possible high levels of BaO, SrO and CaO, a high PbO content of some 30 to 58% by weight is absolutely imperative.
With an nd of from 1.62 to 1.85, the optical glass materials described in document JP 85-221338 A likewise exhibit relatively high refractive indices, the glass materials containing at least 1 to 50% by weight B2O3 and, if only optionally, high levels of BaO (0 to 50% by weight), SrO (0 to 40% by weight), CaO (0 to 30% by weight) and PbO (0 to 30% by weight). B2O3 reduces the chemical-durability of the glass materials.
This also applies to the optical glass materials described in document SU 97 56 17 which, in addition to a B2O3 content of from 2 to 90% by weight, also include CaO (5 to 10% by weight).
The borosilicate glass materials described in document U.S. Pat. No. 4,562,161 likewise exhibit the said optical range, but contain at least 3% by weight B2O3 and at least 4% by weight TiO2, TiO2 having an adverse effect on the transmission, in particular in the UV range.
German document DE 97 33 50 describes optical glass materials which cover a relatively wide range of refractive indices (nd 1.43 to 1.77) and Abbe numbers (vd 28 to 67) and therefore have broad compositional ranges. In addition to the strikingly high optional content of up to 50% by weight PbO, the glass materials contain from 0.2 to 30% by weight TiO2 and from 1.0 to 12% by weight F.
In addition to the principal component SiO2, the optical glass materials described in document JP 85-122747 contain a relatively high level of from 5 to 30% by weight CaO, and the high Abbe numbers vd which can be achieved lie in the range from 51 to 57.
The cesium-containing optical glass materials described in document DE 22 63 501 C2 likewise cover a range of relatively high Abbe numbers vd of from 50 to 60. The component Cs2O, which is employed in an amount of between 5 and 82.5% by weight, makes the glass material considerably more expensive.
The Al2O3-containing (4-16% by weight) glass materials with a relatively low alkaline metal content (Na2O+K2O 1 to 9.5% by weight) described in document JP 07 06 18 36 A are not used as optical glass materials, but rather for specific magnetic applications which require the components FeO2 and MnO2 which are described.
In the glass material which can be doped with Er2O3 according to EP 0 673 892 A2, which contains from 1.5 to 4% by weight Al2O3, B2O3 is replaced by PbO and/or P2O5 in order to improve the spectroscopic properties as optical intensifiers.
The use of ZrO2, such as for example in the spectacles glass materials described in U.S. Pat. No. 2,433,833 (2 to 15% by weight ZrO2) and GB 2 233 781 A (5.71 to 23.15% by weight ZrO2) and in the TiO2-containing optical glass materials described in EP 0 287 345 A1 (at least 2.3% by weight ZrO2), increases the temperature required to melt the glass materials.
The optical glass materials described in JP 82 22 139 A contain, inter alia, 52 to 75% by weight SiO2, 5 to 25% by weight ZnO and 3 to 15% by weight Li2O. However, the essential feature of these glass materials is not the compositional range, but rather the refractive index which is altered by ion exchange in an NaNO3 molten material.
The document JP 87-12633 A describes optical glass materials with gradually adjustable refractive indices; in these materials, substantially cesium undergoes ion exchange against zinc or against other divalent elements. The glass materials are distinguished in particular by their cesium content of approximately 2.86-66.25% by weight Cs2O.
The object of the invention is therefore to provide optical glass materials with a refractive index nd of from 1.52 to 1.66 and an Abbe number vd of from 35 to 54 which exhibit high transmission, high crystallization stability and good melting and processing properties and can be produced at low cost.
This object is achieved in zinc-containing optical glass materials with a refractive index of 1.52xe2x89xa6ndxe2x89xa61.66 and an Abbe number of 35xe2x89xa6vdxe2x89xa654, characterized by the following composition (in % by weight based on oxide) of: SiO2 38-58, ZnO 0.3-42, PbO 0- less than 30, ZnO+PbO 20-55, Li2O 0- less than 3, Na2O 0-14, K2O 0-12, Li2O+Na2O+K2O xe2x89xa72, F 0-3, MgO 0-6, CaO 0-xe2x89xa75, SrO 0-6, BaO 0- less than 0.9, B2O3 0- less than 1, Al2O3 0- less than 1.5 and ZrO2 0- less than 2 and, if appropriate, refining agents in the customary amounts.
The glass materials contain, as network former, from 38 to 58% by weight SiO2; if the SiO2 content rises to above 58% by weight, the melting temperature rises disadvantageously while the refractive indices are significantly reduced.
In addition to SiO2, ZnO, in an amount of from 0.3 to 42% by weight, forms the principal component of the glass materials. Together with PbO, the level of which is restricted to from 0 to  less than 30% by weight, ZnO acts as both a network former and a network modifier.
The total ZnO and PbO content is in the range from 20 to 55% by weight. Preferably the total ZnO and Pbo content is in the range from  greater than 21 to 55% by weight. With high ZnO and PbO contents, high refractive indices nd are established. Varying the ZnO to PbO ratio is used to set the Abbe numbers; low Abbe numbers are achieved with high levels of PbO.
The fact that the glass materials are easy to melt is attributable to their total Li2O, Na2O and K2O content of at least 2% by weight, the Li2O content being restricted to 0 to  less than 3% by weight, the Na2O content being restricted to 0 to 14% by weight, and the K2O content being restricted to 0 to 12% by weight.
It is possible to add small amounts of Al2O3 (0 to  less than 1.5% by weight) and B2O3 (0 to  less than 1% by weight) which likewise reduce the melting temperatures. However, excessively high Al2O3 contents cause the meltability to deteriorate. Elevated B2O3 contents have an adverse effect on the chemical durability, in particular with respect to media containing alkaline metals.
High-melting components are substantially eliminated. For example, the ZrO2 content is limited to 0 to  less than 2% by weight.
In subordinate amounts, the glass materials, in particular in order to fine tune their optical properties, may contain from 0 to 6% by weight MgO, from 0 to  less than 5% by weight CaO, from 0 to 6% by weight SrO and/or from 0 to  less than 0.9% by weight BaO, the glass materials preferably containing from 0 to  less than 0.5% by weight BaO and particularly preferably being substantially free from BaO. The total CaO and SrO content is preferably in the range from 0 to 5% by weight.
Furthermore, from 0 to 3% by weight F is provided. The ranges of the optical data exhibited by the glass materials are restricted in accordance with the restrictions on the particular ranges of the components.
For example, glass materials with refractive indices nd of from 1.54 to 1.64 and Abbe numbers vd of from 40 to 52 are obtained within the following compositional range (in % by weight based on oxide): SiO2 39-54, ZnO 12-41, PbO 6-22, ZnO+PbO 31-52, Li2O 0- less than 3, Na2O 0-13, K2O 0-11, Li2O+Na2O+K2O xe2x89xa72, F 0-3, MgO 0-6, CaO 0- less than 5, SrO 0-6, BaO 0- less than 0.9, B2O3 0- less than 1, Al2O3 0- less than 1.5 and ZrO2 0- less than 2.
Glass materials with refractive indices nd of between 1.56 and 1.63 and Abbe numbers vd of between 42 and 52 are within the following compositional range (in % by weight based on oxide): SiO2 40-55, preferably SiO2 40-53, ZnO 26-41, preferably ZnO 28-41, PbO 1-16, ZnO+PbO 31-48, Li2O 0- less than 3, Na2O 0-12, K2O 0-10, Li2O+Na2O+K2Oxe2x89xa72, F 0-3, MgO 0-6, CaO 0- less than 5, SrO 0-6, BaO 0- less than 0.9, B2O3 0- less than 1, Al2O3 0- less than 1.5 and ZrO2 0- less than 2.
Refractive indices nd from 1.60 to 1.63 and Abbe numbers vd of from 43 to 47 are obtained with glass materials within the following compositional range (in % by weight based on oxide): SiO2 40-47, ZnO 32-41, PbO 5-14, ZnO+PbO 40-48, Li2O 0- less than 3, Na2O 0-14 preferably 0-13 and particularly preferably 0-12, K2O 0-12 preferably 0-11 and particularly preferably 0-10, Li2O+Na2O+K2Oxe2x89xa72, F 0-3, MgO 0-6, CaO 0- less than 5, SrO 0-6, BaO 0- less than 0.9, B2O3 0- less than 1, Al2O3 0- less than 1.5 and ZrO2 0 - less than 2.
Glass materials from the compositional range (% by weight based on oxide): SiO2 41-50, ZnO 30-40, PbO 0-1, ZnO+PbO 31-41, Li2O 0- less than 3, Na2O 0-11, K2O 0-10, Li2O+Na2O+K2O xe2x89xa72, F 0-3, MgO 0-6, CaO 0- less than 5, SrO 0-6, BaO 0- less than 0.9, B2O3 0- less than 1, Al2O3 0- less than 1.5 and ZrO2 0- less than 2, have refractive indices nd of between 1.57 and 1.59 and Abbe numbers vd of between 48 and 52.
To vary and precisely adapt the optical properties, the glass materials may contain up to 2.5% by weight Cs2O and/or up to 5% by weight Rb2O, La2O3, Y2O3 and/or GeO2.
The optical glass materials claimed preferably have a very high pure transmission of at least 0.98, determined for a wavelength of 400 nm and a specimen thickness of 25 mm.
A further advantage of the glass materials according to the invention is their neutral colour. Therefore, if preferably up to 8% by weight of colouring components are added to the glass compositions described, the characteristic spectrum of the colouring components used, such as for example CuO, Cr2O3, CoO, Fe2O3, MnO, NiO and/or V2O5, are particularly valid.
Optical glass materials which have been coloured in this way are used, for example, as optical filters.
To improve the quality of the glass, it is possible for one or more known refining agents to be added in the customary amounts to the batch, in order to refine the glass material. As a result, the glass material has a particularly good internal quality in terms of its freedom from bubbles and cords.
Preferred refining agents are Sb2O3 and/or As2O3 the level of which should be restricted to at most 1% by weight, preferably 0.5% by weight.
In addition to the desired optical properties, the glass materials according to the invention have the following qualities:
It has been possible to substantially minimize the use of the previously customary but relatively expensive glass components BaO, SrO and CaO. In particular, it was possible to further minimize the use of the critical components BaO and PbO.
This was achieved by introducing the inexpensive, ecologically tolerable component ZnO.
Furthermore, the glass materials exhibit good crystallization stability; furthermore they are not only easy to process, but also can be melted with ease.
The glass materials exhibit a sufficiently high chemical durability. The chemical durability is important in particular for the further machining of the materials, for example grinding and polishing.
The above-discussed embodiments of the present invention will be described further hereinbelow. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is, the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.